Some internal combustion engines disposed in vehicles are provided with an air-fuel ratio control system that includes an O.sub.2 sensor and a control means. The O.sub.2 sensor, which serves as an exhaust sensor, is located in an exhaust path of the internal combustion engine. The control means provides feedback control such that an air-fuel ratio achieves its target value in accordance with a detection signal which is sent out from the O.sub.2 sensor.
Japanese Laid-Open Patent No. 61-250355 discloses this type of air-fuel ratio control system for an internal combustion engine. The air-fuel ratio control system disclosed therein is provided with a correcting means which calculates a difference between a first sample time and a second sample time, and corrects the air-fuel ratio in accordance with the difference. The first sample time corresponds to a period of time that elapses between the moment an inversion from a rich air-fuel ratio to a lean air-fuel ratio in an air/fuel mixture occurs, and the moment the inversion is detected by the exhaust sensor. The second sample time indicates a period of time after an inversion from a rich air-fuel ratio to a lean air-fuel ratio in the air/fuel mixture occurs until the inversion is detected by the exhaust sensor.
Other internal combustion engines disposed in vehicles provide air-fuel ratio control systems that include a first O.sub.2 sensor, a second O.sub.2 sensor, and a control means. Serving as exhaust sensors, the first O.sub.2 sensor and the second O.sub.2 sensor are located respectively on an upstream side and a downstream side of a catalyzer which is positioned in an exhaust path of the internal combustion engine. The control means effects feedback control such that an air-fuel ratio achieves its target value in accordance with first and second detection signals which are sent out respectively from the first and second O.sub.2 sensors.
Japanese Laid-Open Patent No. 61-192825 discloses this latter type of air-fuel ratio control system for an internal combustion engine. This air-fuel ratio control system has first and second O.sub.2 sensors positioned respectively upstream and downstream from a catalyzer in an exhaust path of the internal combustion engine in order to effect air-fuel ratio feedback control. The first exhaust sensor, which is positioned upstream from the catalyzer in the exhaust path, is located in a cylinder head of the internal combustion engine.
A problem with exhaust sensors is that variations in a detection signal from an exhaust sensor make it difficult to provide high-precision feedback control such as to match an air-fuel ratio with its target value. Such variations result from manufacturing non-uniformities, deterioration during use, and the like.
In order to overcome the above problem, an air-fuel ratio control system having two sensors and a control means has been proposed, as disclosed in aforesaid Japanese Patent No. 61-192825. More specifically, a first O.sub.2 sensor and a second O.sub.2 sensor are located respectively at an upstream portion and a downstream portion of-an exhaust path of the internal combustion engine with respect to a catalyzer which is positioned in the exhaust path. The control means effects feedback control such as to match an air-fuel ratio with its target value in accordance with a first detection signal and a second detection signal which are sent out respectively from the first O.sub.2 sensor and the second O.sub.2 sensor.
The air-fuel ratio control system disclosed in aforesaid Japanese Patent No. 61-192825 provides a first feedback control in response to the first detection signal from the first O.sub.2 sensor, in order to match an air-fuel ratio with its target value. Further, the first feedback control is controlled so as to be corrected in accordance with the second detection signal from the second O.sub.2 sensor. However, the O.sub.2 sensors are subject to manufacturing non-uniformities and deterioration as previously mentioned. As shown in FIG. 15, a further problem resulting from the above is that variations occur in the reaction time and voltage of a detection signal which an O.sub.2 sensor sends out in accordance with an air-fuel ratio in an air/fuel mixture. Foe example, a curve "a" represents O.sub.2 sensor variations for a product having an upper reaction limit, curve "b" represents O.sub.2 sensor variations for a normal product having a central value and curve "c" represents O.sub.2 sensor variants for a deterioration-resistant product having a lower reaction limit. The variation in the detection signal from the O.sub.2 sensor introduces a change in a reaction cycle, as shown in FIG. 16, which results in changes in the corrected quantity FAF and time of feedback control. A problem arising therefrom is that an actual air-fuel ratio considerably swings with reference to .lambda.=1. For this reason, high-precision feedback control cannot be provided to match an air-fuel ratio with a target value which is required for the catalyzer to improve its clean-up efficiency. As a result, the air-fuel ratio falls out of the target value. This disadvantageously reduces the clean-up efficiency of the catalyzer, while increasing the amount of noxious components in the exhaust emissions.
To obviate the above-described drawbacks, the present invention provides an air-fuel ratio control system for use in an internal combustion engine, in which an exhaust sensor is located in an exhaust path of the internal combustion engine and a control means is provided for effecting feedback control to match an air-fuel ratio with its target value in accordance with a detection signal sent out from the exhaust sensor. The control means is provided with a correction section which, upon fulfillment of predetermined implementation conditions of correction determination, detects a response time that elapses from the moment a change in an operating state of the internal combustion engine occurs, to the moment the detection signal from the exhaust sensor achieves a determination signal value. The correction section governs the feedback control of the air-fuel ratio, which has been effected by means of the exhaust sensor, in accordance with the detected response time so as to correct the feedback control.
Pursuant to the system of the present invention, when predetermined implementation conditions of correction determination are fulfilled, the correction section, which is provided in the control means, detects the response time which elapses after the occurrence of a change in an operating state of the internal combustion engine until the detection signal sent out from the exhaust sensor achieves a determination signal value. In accordance with the detected response time, the correction section governs air-fuel ratio feedback control, which has been effected by means of the exhaust sensor, so as to correct the air-fuel feedback control. In this way, the use of the single exhaust sensor can correct variations in the detection signal from the exhaust sensor, which variations result from manufacturing non-uniformities, deterioration during use, or the like, of the exhaust sensor. As a result, high-precision air-fuel ratio feedback control is achievable.